An electromagnet, or lifting magnet, is a large inductor enclosed in a steel or cast iron case. The electromagnet is used primarily in the steel industry, including scrap processing and manufacturing, for material handling purposes. Most magnets are supplied power from a generator or rectifier at 230 VDC through a magnet controller. Typical industrial magnets range in size from 39 inches to over 100 inches in diameter with full load currents as high as 350 amperes, or more.
As inductors, electromagnets store energy. When a magnet is energized, energy is stored in a magnetic field that is created by the current in the magnet windings. When the supply voltage is removed, the magnetic field begins to collapse. A very important fact is that the voltage across an inductor can change instantly, but the current through an inductor cannot change instantly. Moreover, an inductor opposes any change in current. Therefore, as the field collapses, its polarity is reversed and the inductance becomes a source utilizing the stored energy in an effort to maintain the current.
Because an electromagnet stores energy it must be turned off in a very controlled fashion. The stored magnet energy must be dissipated in such a way that the voltage created within the magnet and at its terminals does not reach destructive levels. Typically, this is accomplished through the magnet controller. When a magnet is de-energized by a controller, the magnet terminals are shorted with a low impedance device such as small value, high wattage, resistor. The discharge terminal voltage is therefore held very low by shunting the magnet current that is being maintained by the collapsing magnetic field, through the discharge resistor. When the magnet terminals are not connected to a resistor, such as when the magnet is open circuited, the terminal resistance can approach infinity. The magnetic field must still dissipate its stored energy. The terminal voltage will therefore rise rapidly, also approaching infinity. The voltage within the magnet will eventually become high enough to cause the insulation within the magnet to fail. When the insulation fails it provides a low resistance path for the magnet current to be maintained. The magnetic field is therefore able to dissipate its stored energy. The insulation previously protected the magnet from short circuits. Once the insulation is overcome by the extremely high internal voltages, the magnet is destroyed. A magnet can easily become open circuited during operation and therefore become subjected to the above-described destructive voltage levels. This frequently occurs when the power cables are accidentally cut or pulled from the magnet while it is energized.
A related safety problem occurs when a worker, thinking the magnet is not energized, intentionally disconnects the electromagnet power cables, for example by manually pulling apart a cable connector. Under these conditions, an enormous arcing voltage up to 60,000 or more volts may occur at the connector and result in serious injury or death to the worker.
In the past, others have attempted to solve the above magnet destruction and personnel safety problems using a spark gap surge arrester. This device consists of two sharply pointed electrodes connected across the magnet terminals. The electrodes are spaced apart at a distance that causes the air gap between them to break down at elevated voltages and discharge the magnet's stored energy in the arc voltage that occurs. This is a crude device and the voltage cannot be easily determined or controlled. It is also easily susceptible to damage. Other typical surge arrester type devices have also been tried but without success. They either could not handle the power dissipation levels or were too large to be installed on the magnet. Thus, prior art with regard to protecting electromagnets and workers from the effects of the very high voltage levels created by an open discharge path is unsatisfactory due to inherent design limitations.